The present invention relates generally to a method and system for making metallic iron nodules (NRI) with reduced CO2 emissions. Metallic iron nodules have been produced by reducing iron oxide such as iron ores, iron pellets, and other iron oxide sources. Various such methods have been proposed so far for directly producing metallic iron nodules from iron ores or iron oxide pellets by using reducing agents such as coal or other carbonaceous material.
Various types of hearth furnaces have been described and used for direct reduction of metallic iron nodules (NRI). One type of hearth furnace used to make NRI is a rotary hearth furnace (RHF). The rotary hearth furnace is partitioned annularly into a drying/preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the supply location and the discharge location of the furnace. An annular hearth is supported rotationally in the furnace to move from zone to zone carrying reducible material the successive zones. In operation, the reducible material comprises a mixture of iron ore or other iron oxide source and reducing material such as carbonaceous material, which is charged onto the annular hearth and initially subject to the drying/preheat zone. After drying and preheating, the reducible material is moved by the rotating annular hearth to the reduction zone where the iron ore is reduced in the presence of the reducing material, and then to the fusion zone where the reduced reducible material is fused into metallic iron nodules, using one or more heating sources (e.g., natural gas burners). The reduced and fused NRI product, after completion of the reduction process, is cooled on the moving annular hearth in the cooling zone to prevent reoxidation and facilitate discharge from the furnace. Another type of furnace used for making NRI is the linear hearth furnace such as described in U.S. Pat. No. 7,413,592, where similarly prepared mixtures of reducible material are moved on moving hearth sections or cars through a drying/preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the charging end and discharging end of a linear furnace while being heated above the melting point of iron.
A limitation of these furnaces and the methods of operating them has been their energy efficiency. The iron oxide bearing material and associated carbonaceous material generally had to be heated in the furnace from near ambient temperature to about 2500° F. (1370° C.), or higher, in order to reduce the iron oxide and produce metallic iron nodules (NRI). Additional energy was also consumed in heating the moving hearth, which may have cooled in transit between the discharging end and the charging end of the furnace.
The reduction process has generally required propane, methane, natural gas or coal to be burned to produce the heat necessary to heat the iron oxide bearing material and associated carbonaceous material to the temperatures necessary to reduce and fuse the iron oxide and produce a metallic iron material. Furthermore, the reduction process involved production of volatiles in the furnace that had to be removed from the furnace and secondarily combusted to avoid an environmental hazard, which added to the energy needs to perform the iron reduction. See, e.g., U.S. Pat. No. 6,390,810.
In addition to volatiles, nitrogen, carbon dioxide, and other exhaust gases were produced in the reduction and fusion processes. The carbon dioxide produced was typically mixed with nitrogen and other exhaust gases and not well adapted to being captured and processed by sequestration. Additionally, the exhaust gases produced often required additional scrubbing and other processing prior to release into the environment. Needed is a linear hearth furnace that reduces and conserves the energy required to reduce the iron oxide bearing material to metallic iron, while also reducing the carbon emissions to the environment.
A method of making metallic iron nodules with reduced CO and CO2 emissions is disclosed that comprises the steps of:                a. assembling a linear hearth furnace having an entry portion and an exit portion, at least a conversion zone and a fusion zone and a moving hearth adapted to move reducible iron bearing material through the furnace on contiguous hearth sections,        b. assembling a shrouded return substantially free of air ingress extending adjacent at least the conversion and fusion zones of the furnace through which hearth sections can move from adjacent the exit portion to adjacent the entry portion of the linear hearth furnace;        c. transferring the hearth sections from the linear hearth furnace to the shrouded return adjacent the exit portion;        d. reducing reducible material in the linear hearth furnace to metallic iron nodules; and        e. transporting gases from at least the fusion zone to the shrouded return to heat the hearth sections while in the shrouded return.        
The method of making metallic iron nodules may include the step of delivering commercially available O2 gas to the conversion zone and fusion zone of the linear hearth furnace to reduce and fuse the reducible iron bearing material to metallic iron nodules and form CO2 gas along with other exhaust gases. Oxygen may be mixed with combustible fuels, in addition to the fluids from the volatiles, so that a CO2 gas is produced adapted for sequestration. The oxygen may also be mixed with other gases such as flue gas, carbon dioxide, or nitrogen to reduce the flame temperature and produce a gas with greater mass to convey heat through the furnace for more efficient reduction and fusion. In addition, at least a portion of the CO2 and/or flue gas exhausted from the linear hearth furnace may be cleaned to produce a commercially viable CO2 gas stream.
The method of making metallic iron nodules may comprise the step of directing CO2 and/or flue gas from the conversion and fusion zones of the linear hearth furnace into the shrouded return to be used in heating the hearth sections during return to the entry portion of the furnace. Optionally, a portion of the flue gases may be circulated to a gasifier.
The method may include, prior to conversion and fusion of the reducible material in the linear hearth furnace, drying and preheating the reducible material in or prior to the linear hearth furnace. Alternatively, the method of making metallic iron nodules may include charging the hearth sections before or after entry into the shrouded return so as to heat reducible material as well as the hearth sections in the shrouded return. The method may also include drying and preheating the reducible material in the shrouded return without substantial fluidization of volatiles in the reducible material. At least some of the carbon dioxide and other gas from the shrouded return may also be mixed with oxygen or combustible fuels and delivered to the conversion or fusion zones of the furnace to provide heat to reduce and form metallic iron bearing material in the furnace. Alternatively or in addition, volatiles in the reducible material may be fluidized during the drying and preheating in the shrouded return and may be transferred to the conversion and/or fusion zones of the linear hearth furnace for combustion.
The method of making metallic iron nodules may further comprise the step of providing a transfer guide adapted to transfer the hearth sections between the linear hearth furnace and shrouded return at both the entry portion and the exit portion of the furnace.
Also disclosed is a system for making metallic iron nodules with reduced CO and CO2 emissions comprising:                a. a linear hearth furnace having an entry portion and an exit portion, at least a conversion zone and a fusion zone, and a moving hearth with a plurality of hearth sections adapted to move reducible iron bearing material through the linear hearth furnace on a guide, such as rails;        b. a shrouded return positioned adjacent the linear hearth furnace through which the hearth sections can move on a guide, such as rails, from adjacent the exit portion to adjacent the entry portion of the linear hearth furnace;        c. passageways adapted to transport gases generated in at least the fusion zone of the furnace to the shrouded return; and        d. transport devices adapted to transport the hearth sections from the exit portion of the furnace to the shrouded return and from the shrouded return to the entry portion of the furnace.        
Additionally, a drying/preheat zone may be provided in or adjacent the shrouded return. Such drying/preheat zone may be provided in whole or in part in the shrouded return with a passageway adapted to transfer volatiles from the drying/preheat zone to the conversion zone or fusion zone. The shrouded return may include baffles adapted to direct the flow of gases and improve heat transfer from the gases to the hearth sections
The system may include a gasifier adapted to produce syn-gas, and at least one gas passageway capable of directing gases from the linear hearth furnace and/or the shrouded return to the gasifier. Alternatively or in addition, the system may include a scrubber adapted to produce a commercially viable CO2 gas stream and at least one gas passageway capable of directing gases from the linear hearth furnace and/or the shrouded return to the scrubber.